Bifocal lenses are a type of eyeglasses that feature two different optical powers in one lens. The top part of the lens corrects distance vision, while a smaller portion at the bottom of the lens corrects near vision. The two viewing zones are separated by a visible line in traditional bifocal lenses. The line is visible when looking closely at the lens, and it marks the boundary between the two different viewing zones. Bifocal lenses are most commonly prescribed to people over the age of 40 to help compensate for the natural degradation of vision due to the aging process.

There are different shapes of the segment devoted to near-vision correction in bifocal lenses, including a half-moon, a round segment, and a narrow rectangular area. When wearing bifocal lenses, you look up and through the distance portion of the lens when focusing on points farther away, and you look down and through the bifocal segment of the lens when focusing on reading material or objects within 18 inches of your eyes.

Modern-day alternatives to traditional bifocal glasses include "no-line" or "progressive" lenses. Progressive lenses are no-line multifocal lenses with up to three viewing zones for distance, intermediate, and near vision. They provide a seamless no-line lens design and clear vision at different distances.